The grammar references I have read including my Russian textbook say на should be followed either by the accusative or prepositional case. But when translating "for lunch" and "for supper", the usages on context.reverso.net all seem to be "на обед" and "на ужин".
Can anyone explain?
In the russian language, "завтрак", "обед" and "ужин" have the same spelling in nominative and accusative cases.
And in your examples ("на завтрак", "на обед" and "на ужин") these words are in accusative case.
На обед and на ужин are accusative, not nominative.
Accusative is quite a peculiar case in Russian.
In singular, its forms have merged with nominative for most nouns except those in -а, -я and a couple more exceptions.
In plural, even the nouns which do have a distinct accusative form in singular, lose it to genitive or nominative, depending on whether the noun is animate or not.
Except for a couple of edge cases, nominative cannot be used with prepositions in Russian at all. When you are seeing something that looks like a nominative after a preposition, consult with the declension table and you'll probably find the same form used in some other case.
